1. Field of the Invention
The present invention relates to an image processing apparatus which determines whether or not to perform smoothing in accordance with the state of a target pixel and its peripheral area and performs smoothing on the target pixel, and a control method of the image processing apparatus.
2. Description of the Related Art
Recently, in the field of image forming apparatuses using electrophotography, various smoothing methods for jaggy correction of an outline, using oversampling or replacement by pattern matching and the like, have been considered.
In the smoothing method of replacing a pixel using pattern matching, a plurality of pixels in the neighborhood of a target pixel are referred, and if the pattern consisting of these plural pixels is a predetermined pattern, correction is executed on the target pixel subjected to forming. According to this method, if the reference area taken from the neighborhood of the target pixel is small, for instance, it is possible to recognize that the target pixel is a part of a curved line, but it is impossible to recognize what curvature the curved line has. Particularly since an outline close to a horizontal or vertical line cannot be detected, it is impossible to perform appropriate correction in accordance with a curvature of the outline. To perform the most appropriate correction in accordance with a curvature of an outline, it is necessary to take a relatively large reference area. However, if pattern matching is to be performed entirely on the large reference area, the size of the comparison circuit will be enlarged, resulting in a high cost.
In the technique disclosed in U.S. Pat. No. 4,847,641, only part of a reference target area is compared with a matching pattern and the remaining portion is disregarded. According to this method, while a large reference target area is taken, the area to be actually compared with a matching pattern is limited to a portion, thereby making it possible to keep the circuit size small. However, since the entire reference target area is not referred in pattern matching, in a case of a halftone image or in a case where pixels are dense, image quality may deteriorate as a result of correction.
In the technique disclosed in U.S. Pat. No. 5,465,157, small sections each of which consisting of a plurality of pixels are extracted, and based on the state of the plurality of extracted small sections, it is determined whether the image is a halftone image or a dense image. According to this method, the aforementioned problem of image quality deterioration can be reduced.
Meanwhile, for a method of comparing a reference area with a matching pattern using a small circuit, a recently widespread technique is to execute the comparison while the matching pattern or the reference area is converted symmetrically with respect to horizontal, vertical, and rotational axes.
As mentioned above, image quality deterioration in halftone images or dense images can be reduced by a method or apparatus which makes use of characteristic reference of small sections. However, in these conventional techniques, when comparison is executed between a reference target pixel and a matching pattern utilizing a symmetric property, pixel array conversion is performed. In order to support the pixel array conversion, it is necessary to register and compare many characteristic patterns of small sections which are different from each another. For this reason, the circuit size may increase and comparison may not be performed efficiently.